The present invention is directed to voltage dependent capacitance circuits, commonly known as varactors, and especially to voltage dependent capacitance circuits that exhibit substantially linear capacitance variance over a voltage range. In its preferred embodiment, the voltage dependent capacitance circuit of the present invention is implemented using metal oxide silicon (MOS) technology.
Nearly all radio frequency (RF) or other analog systems require a phase locked loop (PLL). A PLL is a somewhat complicated device that requires a control element almost always embodied in a type of voltage dependent capacitance device, or varactor, to control the frequency of a VCO (voltage controlled oscillator). Various fabrication technologies may be employed to produce the necessary VCO, including by way of example and not by way of limitation, Schottky diodes and CMOS (complementary metal oxide silicon) processes that include using MOS technology.
Schottky diodes have non-linear capacitance characterisitics. They are commonly used in designing VCO devices, but are not available in CMOS processes. For that reason when one employs CMOS processes an MOS varactor must be employed. An MOS varactor exhibits an even more non-linear voltage-to-capacitance characteristic than a Schottky diode. As a result, a VCO produced using CMOS processes exhibits a very non-linear voltage-to-frequency change. As a consequence, a control system (e.g., a PLL) using a VCO implemented using CMOS processes requires a significant phase margin to allow for variation from linearity in response by the VCO to variations in voltage.
A varactor implemented using CMOS processes (hereinafter referred to as an MOS varactor) may have to be restricted to a small or narrow range of capacitance output to approximate an acceptably linear response to voltage variations.
Another important consideration is that MOS varactors are sensitive to process variations in their manufacture, such as temperature variations, material variations and other processing variations.
There is a need for a varactor apparatus and method that permit substantially linear capacitive response over a relatively wide range of voltage input variation.
In particular there is a need for a varactor apparatus that is manufactured using CMOS processes and permits substantially linear capacitive response over a relatively wide range of voltage input variation.